Nitride semiconductor laser device and wafer

ABSTRACT

Provided is a nitride semiconductor laser device that is reduced in capacitance to have a better response. The nitride semiconductor laser device includes: an active layer; an upper cladding layer which is stacked above the active layer; a low dielectric constant insulating film which is stacked above the upper cladding layer; and a pad electrode which is stacked above the low dielectric constant insulating film.

The present application claims priority from Japanese Patent ApplicationNo. 2009-153470 filed on Jun. 29, 2009, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wafer in which a plurality of nitridesemiconductor laser devices to be separated from one another arearranged, and to a nitride semiconductor laser device obtained from thewafer.

2. Description of Related Art

Nitride semiconductor laser devices, which are utilized in the recordingand playing of a Blu-ray disc or the like and other uses, are beingactively researched and developed. For example, to record information ata high density, turning laser light on and off fast is required and alaser device is accordingly driven with short pulses of approximately 20ns. In pulsed operation, where a laser device exhibits a better responsewhen its impedance is smaller, reducing the resistance that is observedduring driving is important, as well as reducing the capacitance of thelaser device. To reproduce information, on the other hand, a laserdevice is required to be reliable and raising the electrostaticdischarge withstand voltage is important for the reliability of thelaser device.

In the manufacture of a nitride semiconductor laser device, thecrystallinity of a layered laser structure is improved commonly bydigging grooves in a substrate before crystals are grown (see JP2005-322786 A and JP 2006-190980 A). The grooves make the layers thickerin its vicinity and, in order to efficiently obtain laser devices thathave identical optical characteristics from a single substrate, ridgesfor optical waveguides are formed at a distance from the grooves. Thismeans that, when a plurality of devices, 102 to 104, are set between agroove 101 a and a groove 101 b as in FIG. 6, which is a top view of awafer 100, one of the devices (in FIG. 6, the device 104) is alwaysreverse to the rest of the devices (in FIG. 6, the devices 102 and 103)in terms of the placement of the ridge. Specifically, the ridge of theone device (a ridge 104 a in FIG. 6) needs to be formed on the left-handside of the device whereas the ridges of the rest of the devices (ridges102 a and 103 a in FIG. 6) are formed on the right-hand side of thedevices, or vice versa.

Two types of devices having different structures are thus fabricatedfrom the wafer 100, and need to be discriminated from each other in, forexample, a device characteristics test. This is because the two types ofdevices have different emission spots, and the point of introduction oflight into an optical fiber or a tester needs to be changed accordinglywhen the devices are tested for emission wavelength or the like in acharacteristics test. A difference in pad electrode shape, for example,can be used to discriminate one type from the other. In this case, thedifference in pad electrode shape needs to be large enough to berecognizable on image in a tester or a chip mounter.

When devices having different structures are to be discriminated fromone another by their pad electrode shapes, the difference in padelectrode shape or pad electrode area between the devices needs to belarge for easier image recognition, as described above. However, varyingthe pad electrode area from one structure to another means that devicesof different capacitances are fabricated from a single wafer, andcreates the following inconvenience.

When a nitride semiconductor laser device is used to record information,a high frequency superposition circuit is usually employed as acountermeasure for optical feedback noise. The high frequencysuperposition circuit needs to be adjusted for each laser device iflaser devices having different capacitances are used. This causes anincrease in cost and a reduction in productivity, and is thereforeimpractical for actual mass production.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above, and an objectof the present invention is therefore to provide a nitride semiconductorlaser device that is reduced in capacitance to have a better response.Another object of the present invention is to provide a nitridesemiconductor laser device that has a high electrostatic dischargewithstand voltage to be improved in reliability. Still another object ofthe present invention is to provide a way to harvest from a single wafera plurality of nitride semiconductor laser devices of differentstructures that have the same capacitance and that can be discriminatedfrom one another on image by their structures.

In order to achieve the above-mentioned object, the present inventionprovides a nitride semiconductor laser device including: an activelayer; an upper cladding layer which is stacked above the active layer;a low dielectric constant insulating film which is stacked above theupper cladding layer; and a pad electrode which is stacked above the lowdielectric constant insulating film.

According to this structure, the use of the low dielectric constantinsulating film lowers the capacitance of the nitride semiconductorlaser device.

In the nitride semiconductor laser device, it is preferable that theactive layer has underlying layers each of which is an n-type layer andeach layer above the active layer is a p-type layer. This is because thep-type layer is higher in resistance.

In the nitride semiconductor laser device, one of SiOF, SiOC, and anorganic polymer may be employed for the low dielectric constantinsulating film.

Further, the present invention provides a nitride semiconductor laserdevice, including: an active layer; an upper cladding layer which isstacked above the active layer; a high dielectric constant insulatingfilm which is stacked above the upper cladding layer; and a padelectrode which is stacked above the high dielectric constant insulatingfilm.

According to this structure, the use of the high dielectric constantinsulating film increases the capacitance of the nitride semiconductorlaser device.

In the nitride semiconductor laser device, it is preferable that theactive layer has underlying layers each of which is an n-type layer andeach layer above the active layer is a p-type layer. This is because thep-type layer is higher in resistance.

In the nitride semiconductor laser device, one of an HfO₂-based film andan Al₂O₃N-based film may be employed for the high dielectric constantinsulating film.

Further, the present invention provides a wafer including a plurality ofnitride semiconductor laser devices which are to be separated from, oneanother, in which the plurality of nitride semiconductor laser deviceseach include at least one pad electrode that is electrically isolated,and in which, of the plurality of nitride semiconductor laser devices,at least nitride semiconductor laser devices that differ from oneanother in structure have an equal area of a pad electrode to which avoltage is applied, and have different overall pad electrode shapes.

According to this structure, giving the laser devices an equal padelectrode area to which a voltage is applied renders the laser devicesuniform in capacitance, and the difference in overall pad electrodeshape makes the structures of the laser devices discriminable from oneanother through image recognition.

In the wafer, the at least one pad electrode is desirably electricallyisolated by a groove.

Further, the present invention provides a wafer including a plurality ofnitride semiconductor laser devices which are to be separated from oneanother, in which the plurality of nitride semiconductor laser deviceseach have an equal area of pad electrode, and include a low reflectionpart in a part of a surface of the pad electrode which is created bytreatment that lowers reflectance, and in which, of the plurality ofnitride semiconductor laser devices, at least nitride semiconductorlaser devices that have different structures differ from one another ina shape of the low reflection part.

According to this structure, giving the laser devices an equal padelectrode area renders the laser devices uniform in capacitance, and thedifference in low reflection part shape makes the structures of thelaser devices discriminable from one another through image recognition.

In the wafer, the low reflection part is desirably formed from a lowreflection film that is lower in reflectance than the pad electrode.

Further, in the wafer, the low reflection part may be formed byroughening the surface of the pad electrode.

According to the present invention, a nitride semiconductor laser deviceis reduced in capacitance, which gives the nitride semiconductor laserdevice a better response and increases the noise reduction effect of ahigh frequency superposition circuit.

Further, according to the present invention, a nitride semiconductorlaser device is increased in capacitance, which gives the nitridesemiconductor laser device an improved electrostatic discharge damagethreshold (electrostatic discharge withstand voltage) and an enhancedreliability.

Still further, according to the present invention, in harvesting aplurality of nitride semiconductor laser devices of different structuresfrom a single wafer, the laser devices are given a uniform capacitanceto facilitate the adjustment to a high frequency superposition circuitand accordingly lower the manufacture cost, as well as to make thedifferent structures of the laser devices discriminable from one anotheron image.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a sectional view of a nitride semiconductor laser deviceaccording to a first embodiment;

FIG. 2 is a sectional view of a nitride semiconductor laser deviceaccording to a second embodiment;

FIG. 3 is a top view of a wafer according to a third embodiment;

FIG. 4 is a top view of a wafer according to a fourth embodiment;

FIG. 5 is a top view of another wafer according to the fourthembodiment; and

FIG. 6 is a top view of a conventional wafer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The inventors of the present invention has made an extensive research onthe capacitance of nitride semiconductor laser devices and obtained thefollowing knowledge. In AlGaAs-based laser devices and AlGaInP-basedlaser devices, the area of a PN junction of the laser device determinesthe capacitance. This is why a trench structure (created by digging agroove beside a ridge) or a similar measure has been necessary to reducethe capacitance. In a PN junction, what influences the capacitance of anitride semiconductor laser device having a ridge guide device structureis the product of the ridge width plus a few μm and the resonatorlength. The rest are how large a pad electrode area is necessary forwire bonding for external voltage application via a wire on a contactelectrode or the like, and the dielectric constant and thickness of adielectric film under a pad electrode.

This is because an AlGaN layer and a GaN layer in a p-type claddinglayer that is used in a nitride semiconductor laser device are very highin resistance. Usually, a ridge guide laser device has a ridge width ofapproximately 1 to 2 μm and is thinned outside a ridge by etching acladding layer. The thickness of the laser device outside the ridge froman active layer to the top of the cladding layer is approximately 0.1 to0.3 μm. On the outside of the ridge, no contact electrode is usuallyformed and the ridge sides are covered with a dielectric film to guidelight in a lateral direction, which raises the lateral resistanceoutside the ridge. The high resistance causes a voltage applied from theridge to drop as the distance from the ridge increases, with the resultthat no voltage is applied to a PN junction area distanced from theridge. Accordingly, an area distanced from the ridge to which no voltageis applied does not influence the capacitance of the laser device.

A pad electrode made of Ti, Pd, Ni, or Au is on top of a dielectricfilm, which is on top of a p-type AlGAN, GaN, or InGaN layer above a PNjunction. The pad electrode, the dielectric film, and an n-typeelectrode constitute a capacitor. This phenomenon occurs in a PNjunction area that is distanced from a ridge where no voltage is appliedbecause, as mentioned above, a voltage drops as the distance from aridge increases.

When the capacitance at a PN junction near a ridge is given as A and thecapacitance between a pad electrode and an n-type (or a conductivitytype opposite to that of the pad electrode) electrode is given as B, thecapacitance of the laser device is A+B. This capacitance relation isestablished because a capacitor A near a ridge (including the ridge) anda capacitor B between a pad electrode and an n-type (or a conductivitytype opposite to that of the pad electrode) electrode have a parallelrelation.

Four embodiments of the present invention are described below in order.

First Embodiment

FIG. 1 is a sectional view of a nitride semiconductor laser deviceaccording to a first embodiment. The nitride semiconductor laser devicedenoted by 10 is built by stacking an n-type electrode 11, an n-type GaNsubstrate 12, an n-type GaN buffer layer 13, an n-type AlGaN claddinglayer 14, an n-type GaN/InGaN light guiding layer 15, a non-dopedGaN/InGaN active layer 16, a p-type AlGaN vaporization preventing layer17, a p-type GaN or AlGaN interlayer 18, a p-type AlGaN cladding layer19, a p-type GaN contact layer 20, a Pd contact electrode 21, a lowdielectric constant insulating film 22, and a Ti/Au pad electrode 23.

The nitride semiconductor laser device 10 is manufactured by firstgrowing, on the n-type GaN substrate 12, by metal organic chemical vapordeposition (hereinafter abbreviated as MOCVD), the n-type GaN bufferlayer 13, the n-type AlGaN cladding layer 14, the n-type GaN/InGaN lightguiding layer 15, the non-doped GaN/InGaN active layer 16, the p-typeAlGaN vaporization preventing layer 17, the p-type GaN or AlGaNinterlayer 18, the p-type AlGaN cladding layer 19, and the p-type GaNcontact layer 20 sequentially.

Next, the Pd contact electrode 21 is formed on the p-type GaN contactlayer 20 and then the stack is etched down to a point in the p-typeAlGaN cladding layer 19 by photolithography and dry etching, to therebyform ridges 24 in a stripe pattern. An SiOF film to constitute the lowdielectric constant insulating film 22 is formed by sputtering to athickness of approximately 100 nm, so as to cover ridge sides. Parts ofthe SiOF film that are above the ridges 24 are removed by etching.

The Ti/Au pad electrode 23 is formed next. The rear surface of then-type GaN substrate 12 is polished to be thinned down to approximately100 μm, and then subjected to rear surface treatment. Thereafter, anNi/Au film or the like is formed as the n-type electrode 11 on thetreated rear surface.

The stack is then cleaved along a plane perpendicular to the ridges 24to form resonator mirrors. A dielectric film (made of Al₂O₃, SiO₂, TiO₂,or the like) is formed on the cleaved surface. The stack is then scribedbetween stripes and broken along the scribe lines into pieces to obtaina desired nitride semiconductor laser device 10.

The capacitance of the nitride semiconductor laser device 10manufactured in this manner is approximately 6 pF, and is lower than thecapacitance of a nitride semiconductor laser device that uses aninsulating film different from the low dielectric constant insulatingfilm 22, which is approximately 10 pF. As a result, the nitridesemiconductor laser device 10 has a better response and enhances thenoise reduction effect brought by a high frequency superposition circuitto a degree that the relative intensity of noise (RIN) at 3 to 5 mW is−125 dB or less (conventionally, −125 dB or less).

The low dielectric constant insulating film 22 uses a low dielectricconstant material in which the dielectric constant k value is, forexample, 5 or less. While an SiOF film is used here as the lowdielectric constant insulating film 22, the same effect is obtained withan SiOC film or an organic polymer film. An organic polymer suitable forthe low dielectric constant insulating film 22 is, for example, aborazine-silicon polymer.

Second Embodiment

FIG. 2 is a sectional view of a nitride semiconductor laser deviceaccording to a second embodiment. The nitride semiconductor laser devicedenoted by 30 is built by stacking an n-type electrode 31, an n-type GaNsubstrate 32, an n-type GaN buffer layer 33, an n-type AlGaN claddinglayer 34, an n-type GaN/InGaN light guiding layer 35, a non-dopedGaN/InGaN active layer 36, a p-type GaN or AlGaN interlayer 37, a p-typeAlGaN vaporization preventing layer 38, a p-type AlGaN cladding layer39, a p-type GaN contact layer 40, a Pd contact electrode 41, a highdielectric constant insulating film 42, and a Ti/Au pad electrode 43.

The nitride semiconductor laser device 30 is manufactured by firstgrowing, on the n-type GaN substrate 32, by metal organic chemical vapordeposition (hereinafter abbreviated as MOCVD), the n-type GaN bufferlayer 33, the n-type AlGaN cladding layer 34, the n-type GaN/InGaN lightguiding layer 35, the non-doped GaN/InGaN active layer 36, the p-typeGaN or AlGaN interlayer 37, the p-type AlGaN vaporization preventinglayer 38, the p-type AlGaN cladding layer 39, and the p-type GaN contactlayer 40 sequentially.

Next, the Pd contact electrode 41 is formed on the p-type GaN contactlayer 40 and then the stack is etched down to a point in the p-typeAlGaN cladding layer 39 by photolithography and dry etching, to therebyform ridges 44 in a stripe pattern. An HfO₂ film to constitute the highdielectric constant insulating film 42 is formed by sputtering to athickness of approximately 100 nm, so as to cover ridge sides. Parts ofthe HfO₂ film that are above the ridges 44 are removed by etching.

The Ti/Au pad electrode 43 is formed next. The rear surface of then-type GaN substrate 32 is polished to be thinned down to approximately100 μm, and then subjected to rear surface treatment. Thereafter, anNi/Au film or the like is formed as the n-type electrode 31 on thetreated rear surface.

The stack is then cleaved along a plane perpendicular to the ridges 44to form resonator mirrors. A dielectric film (made of Al₂O₃, SiO₂, TiO₂,or the like) is formed on the cleaved surface. The stack is then scribedbetween stripes and broken along the scribe lines into pieces to obtaina desired nitride semiconductor laser device 30.

The capacitance of the nitride semiconductor laser device 30manufactured in this manner is approximately 20 pF, and is higher thanthe capacitance of a nitride semiconductor laser device that uses aninsulating film different from the high dielectric constant insulatingfilm 42 which is approximately 10 pF. As a result, the nitridesemiconductor laser device 30 has favorable characteristics in which theelectrostatic discharge damage threshold (electrostatic dischargewithstand voltage) is improved to 150 V or higher (conventionally 70 Vor higher) at a resonator length of 400 μm.

The high dielectric constant insulating film 42 uses a high dielectricconstant material in which the dielectric constant k value is, forexample, 90 or more. While an HfO₂ film is used above as the highdielectric constant insulating film 42, the same effect is obtained withan Al₂O₃N-based film. The Al₂O₃N-based film here refers to an AlON film,an AlO₂N film, an Al₂ON film, and the like.

Third Embodiment

A third embodiment of the present invention uses a processed substratethat is an n-type GaN substrate with grooves formed in a stripe patternin its top face (epitaxy face). The processed substrate is manufacturedby first forming an SiO₂ film or the like on the top face of an n-typeGaN substrate by sputtering evaporation to a thickness of 1 μm, andforming a stripe photoresist pattern by a common lithography processsuch that the photoresist pattern has a width of 5 μm at a resistopening and an interval (cycle) of 400 μm between the center of a stripeand the center of an adjacent stripe.

Next, the SiO₂ film and the n-type GaN substrate are etched by dryetching such as reactive ion etching (RIE) technology to form grooveseach having a depth of 5 μm and an opening width of 5 μm. The SiO₂ filmis then removed with the use of an etchant such as HF, to thereby obtainthe processed substrate.

The deposition by evaporation of SiO₂ is not limited to sputteringevaporation, and electron beam evaporation, plasma CVD, or the like maybe employed instead. The cycle of the resist pattern is not limited to400 μm, which is described above, and may be changed to suit the widthof a nitride semiconductor laser device manufactured. The dry etchingfor digging the grooves may be replaced by wet etching.

FIG. 3 is a top view of a wafer according to the third embodiment. Thewafer, denoted by 50, is broken into nitride semiconductor laser deviceseach of which is built by stacking an n-type electrode, a processedsubstrate, an n-type GaN buffer layer, an n-type AlGaN cladding layer,an n-type GaN/InGaN light guiding layer, a non-doped GaN/InGaN activelayer, a p-type GaN or AlGaN interlayer, a p-type AlGaN vaporizationpreventing layer, a p-type AlGaN cladding layer, a p-type GaN contactlayer, a Pd contact electrode, a dielectric film, and a Ti/MO/Au padelectrode.

The nitride semiconductor laser device 10 is manufactured by firstgrowing, on the processed substrate, by metal organic chemical vapordeposition (hereinafter abbreviated as MOCVD), the n-type GaN bufferlayer, the n-type AlGaN cladding layer, the n-type GaN/InGaN lightguiding layer, the non-doped GaN/InGaN active layer, the p-type GaN orAlGaN interlayer, the p-type AlGaN vaporization preventing layer, thep-type AlGaN cladding layer, and the p-type GaN contact layersequentially.

Next, the Pd contact electrode is formed on the p-type GaN contact layerand then the stack is etched down to a point in the p-type AlGaNcladding layer by photolithography and dry etching, to thereby formridges in a stripe pattern. An SiO₂ film to constitute the dielectricfilm is formed by sputtering to a thickness of approximately 100 nm, soas to cover ridge sides. Parts of the SiO₂ film that are above theridges 51 are removed by etching.

Next, resist is applied and a pad electrode pattern is formed such thatphoto etching leaves the resist in unnecessary areas. Ti/MO/Au padelectrodes are then formed by electron beam evaporation. Unnecessaryparts of the pad electrodes are removed by lift-off to ultimately obtainpad electrodes 52 a to 52 g illustrated in FIG. 3. In FIG. 3, threelaser devices are formed between one groove 56 and another groove 56which are created as a result of stacking the layers on the grooves inthe processed substrate.

The pad electrodes 52 a to 52 g may be formed by another method in whicha Ti/MO/Au film is formed by sputtering or electron beam evaporation onthe entire top face of the wafer 50, resist is applied, and a padelectrode pattern is formed such that photo etching leaves the resist innecessary areas, and unnecessary parts of pad electrodes are removedwith an iodine-based etchant.

The pad electrodes 52 a, 52 d, and 52 g formed on ridges 51 are padelectrodes in which wire bonding is performed, i.e., pad electrodes towhich a voltage is applied. The pad electrodes 52 a, 52 d, and 52 g towhich a voltage is applied are designed to have an equal area in orderto give their laser devices the same capacitance.

The pad electrodes 52 b, 52 c, 52 e, and 52 f, on the other hand, arepad electrodes in which wire bonding is not performed, i.e., padelectrodes to which no voltage is applied, and which are not formed onthe ridges 51. The pad electrodes 52 b, 52 c, 52 e, and 52 f areelectrically isolated by forming grooves in areas between the padelectrodes 52 b, 52 c, 52 e, and 52 f and the pad electrodes 52 a, 52 d,and 52 g to which a voltage is applied (areas where no pad electrode isformed).

Each nitride semiconductor laser device obtained by breaking the waferinto pieces is designed such that all pad electrodes in one devicetogether form a shape different from the overall pad electrode shape ofanother device. For example, the overall shape of the pad electrodes 52a to 52 c, the overall shape of the pad electrodes 52 d to 52 f, and theshape of the pad electrode 52 g differ from one another.

This is because, while devices 53 and 54 have the same structure, adevice 55 has a reverse structure in terms of the placement of theridge. Specifically, the device 55 has a ridge on the left-hand sidewhereas the devices 53 and 54 have ridges on the right-hand side, orvice versa. The coexistence of two types of devices of differentstructures necessitates discrimination between the two types of devicesin, for example, a device characteristics test. Because the two types ofdevices have different emission spots, the point of introduction oflight into an optical fiber or a tester needs to be changed accordinglywhen the devices are tested for emission wavelength or the like in acharacteristics test. The difference in pad electrode shape is used todiscriminate one type from the other through image recognition in atester or a chip mounter.

Accordingly, the overall pad electrode shape needs to be varied suchthat at least the discrimination of the device 55 from the devices 53and 54 is possible. The devices 53 and 54 which have the same structuredoes not need to be discriminated from each other, but, in FIG. 3, wherethe pad electrode 52 b and the pad electrode 52 e have different shapes,are discriminable from each other.

Returning to the description of how the nitride semiconductor laserdevice is manufactured, the rear surface of the processed substrate ispolished to be thinned down to approximately 100 μm, and then subjectedto rear surface treatment. Thereafter, an Ni/Au film or the like isformed as the n-type electrode on the treated rear surface.

The stack is then cleaved along a plane perpendicular to the ridges 51to form resonator mirrors. A dielectric film (made of Al₂O₃, SiO₂, TiO₂,or the like) is formed on the cleaved surface. The stack is then scribedbetween stripes and broken along the scribe lines into pieces to obtaindesired nitride semiconductor laser devices 53 to 55.

The capacitance of the nitride semiconductor laser devices 53 to 55manufactured in this manner is approximately 10 pF. By thus giving thedevices 53 to 55 a uniform capacitance, the adjustment of a highfrequency superposition circuit is facilitated and the manufacture costis reduced by approximately 10%.

With the structure of this embodiment, a plurality of nitridesemiconductor laser devices harvested from a single wafer and havingdifferent structures can have the same capacitance and can bediscriminated from one another by their structures through imagerecognition.

Fourth Embodiment

FIG. 4 is a top view of a wafer according to the fourth embodiment. Thewafer, denoted by 60, is broken into nitride semiconductor laser deviceseach of which is built by stacking an n-type electrode, a processedsubstrate, an n-type GaN buffer layer, an n-type AlGaN cladding layer,an n-type GaN/InGaN light guiding layer, a non-doped GaN/InGaN activelayer, a p-type GaN or AlGaN interlayer, a p-type AlGaN vaporizationpreventing layer, a p-type AlGaN cladding layer, a p-type GaN contactlayer, a Pd contact electrode, a dielectric film, and a Ti/MO/Au padelectrode.

The nitride semiconductor laser device 10 is manufactured by firstgrowing, on the processed substrate, by metal organic chemical vapordeposition (hereinafter abbreviated as MOCVD), the n-type GaN bufferlayer, the n-type AlGaN cladding layer, the n-type GaN/InGaN lightguiding layer, the non-doped GaN/InGaN active layer, the p-type GaN orAlGaN interlayer, the p-type AlGaN vaporization preventing layer, thep-type AlGaN cladding layer, and the p-type GaN contact layersequentially.

Next, the Pd contact electrode is formed on the p-type GaN contact layerand then the stack is etched down to a point in the p-type AlGaNcladding layer by photolithography and dry etching, to thereby formridges 61 in a stripe pattern. An SiO₂ film to constitute the dielectricfilm is formed by sputtering to a thickness of approximately 100 nm, soas to cover ridge sides. Parts of the SiO₂ film that are above theridges 61 are removed by etching.

Next, a Ti/MO/Au film to constitute pad electrodes is formed on theentire top face of the wafer 60 by sputtering or electron beamevaporation. A low reflection film that does not have metallic luster issubsequently formed from SiO₂ by evaporation to a thickness that is notrecognized in image recognition. Resist is applied onto the SiO₂ filmand a pad electrode pattern is formed such that photo etching leaves theresist in necessary areas. Unnecessary parts of the SiO₂ film areremoved by an etchant such as buffered hydrofluoric acid.

Pad electrodes 62 a to 62 c and low reflection films 63 a to 63 d arethus obtained as illustrated in FIG. 4. In FIG. 4, three laser devicesare formed between one groove 64 and another groove 64 which are createdas a result of stacking the layers on the grooves in the processedsubstrate.

The low reflection films 63 a to 63 d may use other materials than SiO₂as long as the reflectance of the employed material is lower than thatof the pad electrodes 62 a to 62 c. Instead of the low reflection films,treatment may be employed for giving a pad electrode a lower reflectancethan that of an unprocessed pad electrode, such as treatment thatroughens a pad electrode surface. This part on a pad electrode where thereflectance is low is called a low reflectance part.

Specifically, the pad electrode surface roughening treatment uses anetchant such as hydrochloric acid, and is performed on a pad electrodesurface after a Ti/MO/Au film to constitute pad electrodes is formed onthe entire top face of the wafer 60 by sputtering or electron beamevaporation, resist is applied onto the Ti/MO/Au film, and a padelectrode pattern is formed such that photo etching leaves the resist innecessary areas. FIG. 5 is a top view of a wafer 60′ in which a padelectrode surface has been roughened. Roughened parts of the padelectrode surface are illustrated as surface-treated parts 65 a to 65 d.

Each nitride semiconductor laser device obtained by breaking the waferinto pieces is designed such that the pad electrode shape viewed fromabove, namely, the shape of an area where the low reflection parts donot overlap each other, differs from one laser device to another. Thiscan be rephrased that the low reflection part shape differs from onelaser device to another.

This is because, while devices 66 and 67 (66′ and 67′) have the samestructure, a device 68 (68′) has a reverse structure in terms of theplacement of the ridge. Specifically, the device 55 has a ridge on theleft-hand side whereas the devices 53 and 54 have ridges on theright-hand side, or vice versa. The coexistence of two types of devicesof different structures necessitates discrimination between the twotypes of devices in, for example, a device characteristics test. Becausethe two types of devices have different emission spots, the point ofintroduction of light into an optical fiber or a tester needs to bechanged accordingly when the devices are tested for emission wavelengthor the like in a characteristics test. The difference in pad electrodeshape is used to discriminate one type from the other through imagerecognition in a tester or a chip mounter.

Accordingly, the apparent shape of the overall pad electrode needs to bevaried by differentiating the shapes of the low reflection parts suchthat at least the discrimination of the device 68 (68′) from the devices66 and 67 (66′ and 67′) is possible. The devices 66 (66′) and 67 (67′)which have the same structure does not need to be discriminated fromeach other but, in FIG. 4 or 5, where the pad electrode 62 a and the padelectrode 62 b have different apparent shapes, are discriminable fromeach other.

Returning to the description of how the nitride semiconductor laserdevice is manufactured, the rear surface of the processed substrate ispolished to be thinned down to approximately 100 μm, and then subjectedto rear surface treatment. Thereafter, an Ni/Au film or the like isformed as the n-type electrode on the treated rear surface.

The stack is then cleaved along a plane perpendicular to the ridges 61to form resonator mirrors. A dielectric film (made of Al₂O₃, SiO₂, TiO₂,or the like) is formed on the cleaved surface. The stack is then scribedbetween stripes and broken along the scribe lines into pieces to obtaindesired nitride semiconductor laser devices 66 to 68 (66′ to 68′).

The capacitance of the nitride semiconductor laser devices 66 to 68 (66′to 68′) manufactured in this manner is approximately 15 pF. By thusgiving the devices 53 to 55 a uniform capacitance, the adjustment of ahigh frequency superposition circuit is facilitated and the manufacturecost is reduced by approximately 10%.

With the structure of this embodiment, a plurality of nitridesemiconductor laser devices harvested from a single wafer and havingdifferent structures can have the same capacitance and can bediscriminated from one another by their structures through imagerecognition.

The first to fourth embodiments may be combined as seen fit. Bycombining two or more embodiments, a hybrid structure such as one havinga low or high dielectric constant insulating film in which padelectrodes are electrically isolated or low reflection parts areprovided on pad electrodes can be created, and the respective effects ofthe embodiments used in combination are obtained as well.

A nitride semiconductor laser device according to the present inventionis applicable to an optical pickup installed in an optical disk drivethat plays and records on an optical disk such as a Blu-ray disc.

1. A nitride semiconductor laser device, comprising: an active layer; anupper cladding layer which is stacked above the active layer; a lowdielectric constant insulating film which is stacked above the uppercladding layer; and a pad electrode which is stacked above the lowdielectric constant insulating film.
 2. A nitride semiconductor laserdevice according to claim 1, wherein the active layer has underlyinglayers each of which is an n-type layer and each layer above the activelayer is a p-type layer.
 3. A nitride semiconductor laser deviceaccording to claim 1, wherein the low dielectric constant insulatingfilm is formed from one of SiOF, SiOC, and an organic polymer.
 4. Anitride semiconductor laser device, comprising: an active layer; anupper cladding layer which is stacked above the active layer; a highdielectric constant insulating film which is stacked above the uppercladding layer; and a pad electrode which is stacked above the highdielectric constant insulating film.
 5. A nitride semiconductor laserdevice according to claim 4, wherein the active layer has underlyinglayers each of which is an n-type layer and each layer above the activelayer is a p-type layer.
 6. A nitride semiconductor laser deviceaccording to claim 4, wherein the high dielectric constant insulatingfilm comprises one of an HfO₂-based film and an Al₂O₃N-based film.
 7. Awafer, comprising a plurality of nitride semiconductor laser deviceswhich are to be separated from one another, wherein the plurality ofnitride semiconductor laser devices each comprise at least one padelectrode that is electrically isolated, and wherein, of the pluralityof nitride semiconductor laser devices, at least nitride semiconductorlaser devices that differ from one another in structure have an equalarea of a pad electrode, to which a voltage is applied, and havedifferent overall pad electrode shapes.
 8. A wafer according to claim 7,wherein the at least one pad electrode is electrically isolated by agroove.
 9. A nitride semiconductor laser device, which is obtained bybreaking the wafer according to claim 7 into pieces.
 10. A wafer,comprising a plurality of nitride semiconductor laser devices which areto be separated from one another, wherein the plurality of nitridesemiconductor laser devices each have an equal area of pad electrode,and comprise a low reflection part in a part of a surface of the padelectrode which is created by treatment that lowers reflectance, andwherein, of the plurality of nitride semiconductor laser devices, atleast nitride semiconductor laser devices that have different structuresdiffer from one another in a shape of the low reflection part.
 11. Awafer according to claim 10, wherein the low reflection part comprises alow reflection film that is lower in reflectance than the pad electrode.12. A wafer according to claim 10, wherein the low reflection part isformed by roughening the surface of the pad electrode.
 13. A nitridesemiconductor laser device, which is obtained by breaking the waferaccording to claim 10 into pieces.